Spinal fixation systems may be used in surgery to align, adjust and/or fix portions of the spinal column, i.e., vertebrae, in a desired spatial relationship relative to each other. Many spinal fixation systems employ a spinal rod, i.e. a spinal fixation element, for supporting the spine and for properly positioning components of the spine for various treatment purposes. The fixation system components, such as vertebral anchors, comprising pins, bolts, screws, and hooks, engage the vertebrae and connect the supporting rod to different vertebrae. The spinal fixation system components can have a predetermined contour that has been designed according to the properties of the target implantation site. Once installed, the spinal fixation system holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time. The size, length and shape of the spinal rod depend on the size, number and position of the vertebrae to be held in a desired spatial relationship relative to each other by the apparatus.
Spinal fixation system components can be anchored to specific portions of the vertebra. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone. Pedicle screw assemblies, for example, have a shape and size that is configured to engage pedicle bone. Such screws typically include a threaded shaft that is adapted to be threaded into a vertebra, and a head portion having a spinal fixation element-receiving portion for receiving, for example, a spinal fixation rod. A set-screw, plug, cap or similar type of closure mechanism is used to lock the spinal rod onto the rod-receiving portion of the pedicle screw. In use, the shaft portion of each screw is threaded into a vertebra, and once properly positioned, the spinal fixation rod is seated through the rod-receiving portion of each screw.
Recently, the trend in spinal surgery has been moving toward providing minimally invasive elements and methods for implanting spinal fixation systems. In some anchor devices, rigid extension elements are attached to the heads of the anchor devices and extend out of the skin incision. These rigid extension elements may be used to control the head of the anchor device to assist the spinal fixation element in entering into the head of the anchor device. The rigid extension elements limit the entry zone of the spinal fixation element into the head of the bone anchor. Thus the placement of the spinal fixation element into the bone anchor head becomes very challenging. This is especially problematic in extended constructs where multiple vertebral bodies are being fixated.
One or more bone anchors may not be inserted immediately adjacent to the spinal fixation element. Additionally, in many instances one or more vertebrae may be out of alignment such that the one or more vertebrae and the inserted bone anchor are not immediately adjacent to the inserted spinal rod or the bone anchors do not have comparable heights. In these cases, since the spinal fixation rod cannot follow a well aligned path, it becomes challenging or impossible to place the spinal fixation rod though each bone anchor. Specifically, in percutaneous or minimally invasive procedures, it is more difficult to adjust a spinal rod using a technique such as bending to make contact between the spinal rod and the bone anchors. It is also more difficult to move such vertically or laterally displaced vertebrae so that the vertebrae may be coupled to the spinal rod.
Additionally, the current extension elements used in MIS procedures cause other logistical issues for the procedure. Extension elements that crowd the incision area add complexity to the operation. The time required managing assembling the extensions elements and disengaging them may be extensive.